Cascode amplifiers that each include an amplifier circuit including two transistors cascode-connected to each other, and a bias circuit applying a bias voltage to the two transistors included in the amplifier circuit have conventionally been used. The amplifier circuit includes the transistor (a common-source device) grounded with a source terminal, and the transistor (a common-gate device) cascode-connected to the common-source device. The bias circuit generates the bias voltage to be applied to the common-source device from a reference current source and the diode-connected transistor. The bias circuit also generates the bias voltage to be applied to the common-gate device by applying the same overdrive voltage as in the amplifier body to a replica circuit that includes two transistors cascode-connected to each other. The configuration described above generates, in the amplifier circuit, a bias current that has a magnitude of the reference current multiplied by the channel width/channel length ratio of the common-source device and the common-gate device.
Under a bias condition appropriate to a linear power amplification, the cascode amplifier described above increases the difference between a gate-source voltage Vgs that is the voltage between the gate terminal and source terminal of the common-source device in the amplifier circuit, and a drain-source voltage Vds that is the voltage between the drain terminal and the source terminal. On the other hand, the gate-source voltage Vgs is equal to the drain-source voltage Vds in the transistor applying the bias voltage to the amplifier circuit because of the diode connection. As described above, it is difficult to set an operating point with a high degree of accuracy according to the reference current and the channel width/channel length ratio because the operating conditions of the amplifier circuit and the bias circuit widely vary in a conventional cascode amplifier. Such deterioration in the accuracy is more pronouncedly caused by the production tolerance of each device and the variation in the power-supply voltage.